Back in the days of film, I had my own darkroom equipment and really enjoyed the work in there. Today, the digital darkroom is the computer. I miss some of the darkroom advantages, but I prefer digital: no more chemicals; fumbling in the dark; or messy clean-ups.
There are many programs to assist you in post-processing, but Adobe's Photoshop is the standard. If you want more than just Photoshop for your work, there are many software companies with plug-ins that allow additional, specialized adjustments. You always want to get the best image possible in the camera, but I still enjoy working on images inside the digital darkroom.

Recently, I was adding a few touches to a friend's image, and she asked me to change her eye color. She wanted to see how she would look with blue or green eyes. Off we go to Photoshop....

When you look at an eye closely, you see that the iris is not a single color, but a mixture of colors with one color dominating. You can't just go into the image and paint in a color if you want the image to look natural. I put in a hue/saturation adjustment layer, with the controls set to master, which causes a color shift in all the colors of the image. I added a mask on the adjustment layer to ensure the eyes were the only things that changed in the image. The black mask hides all the adjustments. Use a paintbrush, set to white, to reveal the adjustments you want to see.

Here are the different versions.

This is really a pretty easy fix, and you can make some very radical color shifts if that is your intent. You can use this technique to change the colors of all kinds of things. Why it could even save your life. What if you were hamming it up with your Army buds, at the North Bridge in Concord, some time in April of '75, just before a famous battle, and now you want to be a part of the rebel cause? Well, you just grab the image and perform a simple adjustment. Your red coat becomes blue, and you are on the winning side. ;-)